1) Field of the Invention
The present invention relates in general to an apparatus for partial combustion of fuel mixtures composed of pulverized bituminous or subbituminous coal and oxidizer gas at or above the ash fusion temperature to generate inflammable exhaust gases like as fuel for boilers. This invention is directed more particularly to such an apparatus in which the fuel mixture is substoichiometrically burned by a pre-combustion chamber in conjunction with a main combustion chamber such that the resultant exhaust gases, mostly deprived of the contained non-combustible substances, which are removed as molten slag, permit to be utilized in the secondary-stage furnace to which the gases are passed from the main combustion chamber.
A further aspect of the present invention is concerned with a transport duct that is interconnected between the primary stage furnace for partial combustion of air-fuel mixtures to generate inflammable raw gases and the secondary-stage furnace for the utilization of the exhaust gases received through the duct from the primary-stage furnace. The duct is designed so as to help reduce the non-combustible by-products contained in the exhaust gases.
2) Description of the Prior Art
Cyclone burners have been known as systems to provide complete combustion of coal, and in universal use with heat exchange equipment such as boilers. A typical cyclone burner consists of a water-cooled horizontal cylinder and a main combustion chamber. Fuel or pulverized coal is first introduced into the cylinder at one end thereof and picked up by a stream of air flowing in a tangential direction to the cylindrical main chamber. Blended into the tangential air stream into the main chamber, the pulverized coal is given rapid swirling motion while it is being burned in the heat generated in the cyclone burner main chamber by a burner unit which is fired in advance to heat the main chamber to proper temperature that insures complete combustion of the fuel.
In the process, the non-combustibles, such as ash, present in the fuel are centrifuged onto the cyclone burner wall to form a film of molten slag on the wall. A small quantity of relatively fine coal particles burn in their flight through the cyclone burner while the vast majority of the coal is large coal particles which are centrifuged onto the wall. These larger particles adhere to the molten slag film on the wall and burn while on the wall. As a result, high-temperature gases completely burned by products, such as carbon dioxides are generated, and are allowed to flow into a furnace. In the furnace, which essentially forms the secondary-stage furnace of a boiler, the completely burned gases are utilized to produce steam in the boiler.
However, these conventional cyclone burners have been found to pose problems. First, reaction in the combustion chamber of the cyclone burners tend to have 10.about.20% of the non-combustible by-products in the air-fuel mixture left suspended in molten stage in the resultant raw gases being passed into the associated secondary-stage furnaces. When the raw gases are further burned in the secondary-stage furnaces, these non-combustibles fall and deposit in their internal bottom. Where the boilers are of the type having a heat convection surface directly installed in their secondary-stage furnace, the non-combustibles as molten slag adhere to the surface, causing undesirable trouble in the system such as contamination and premature wear.
Furthermore, when the raw gases stream into the secondary-stage furnace, part of the non-combustibles in molten state is left adhered to the surface of the baffle, a perforated dividing wall between the cyclone burner and secondary-stage furnace, to form a layer of more or less hardened slag. When the next stream of raw gases bursts passing the baffle, they tend to scrape some of the slag off the baffle surface, and bring it with them into the secondary-stage furnace where the slag deposits at its bottom.
In addition, these cyclone burners are often built too large to insure stable ignition or steady inflammation at desired temperature. Secondly, their designs are such that the combustion chamber operating environment tends to speed reaction, causing the coal to burn into too a rapid expansion of gases to develop a swirling motion. As a result, there would be no enough momentum in the resultant exhaust gases that could enable the non-combustibles present in the gases to be centrifuged onto the combustion chamber wall, making it difficult to permit proper removal of the non-combustibles as molten ash.
U.S. Pat. No. 4,542,704, Braun, discloses another example of a furnace system for combustion of coal by ash removal. The furnace comprises a primary-stage, a secondary-stage and a tertiary-stage furnace in which coal with a high sulfur content is burned in such a manner to reduce the non-combustible particulates and sulfur pollutants present in the resultant exhaust gases. This is achieved by blending into the coal an additive that reacts with sulfur in the first-stage reaction in which the coal is exposed to heat below the ash fusion temperature. The resultant incompletely burned exhaust gases are then further burned in the secondary-stage furnace at or above the ash fusion temperature to generate inflammable raw gases which are caused to undergo complete combustion in the presence of sufficient air to produce steam in the tertiary-stage furnace to which the primary-stage and the secondary-stage furnace are connected.
However, the Braun's furnace also has been proved to suffer from various difficulties. Partial combustion requires that the primary-stage furnace be burned with a set of operating parameters. For example, the amount of air to be blended with the fuel is limited to 75% or below of the required volume to fully burn that fuel. The furnace reaction temperature is maintained at 800.about.1,050 degree Celsius, too low a level to insure stable ignition and sustained combustion. Furthermore, the resultant exhaust gases are relatively low in temperature enough to provide stable complete combustion in the secondary-stage furnace.
In addition, with Braun, if the heat in the secondary-stage furnace fell below rating, the ratio of fuel mixed in the air-fuel mixture used at the primary-stage furnace is increased until the secondary-stage combustion environment reaches the rating. However, this would result in a plunge in the temperature of the primary-stage furnace. When the ratio of air in the mixture is increased to boost the temperature of the resultant exhaust gases, a localized excess of heating occurs in the primary-stage furnace. This would make it impossible to achieve the claimed objects of the Braun system of fusing part of the non-combustibles in the primary-stage combustion and maintaining the secondary-stage combustion environment at or above the ash fusion temperature.